One way in which telecommunication service providers provide high-speed digital communication services is by using digital subscriber line (DSL) technology. In one typical configuration, a high-bit rate digital subscriber line (HDSL2) transceiver unit located in a central office of a service provider communicates with an HDSL2 transceiver unit located at a remote site. The former transceiver unit is also referred to here as an “H2TU-C” and the latter transceiver unit is also referred to here as an “H2TU-R”. The H2TU-C communicates with the H2TU-R over a single twisted-pair telephone line using HDSL2 technology.
The twisted-pair telephone line is typically coupled to the H2TU-R using an isolation transformer. The isolation transformer has a primary winding that is coupled to the twisted-pair telephone line and a secondary winding that is coupled to the signal processing components of the H2TU-R (for example, an analog front end, line driver, etc.). The tip and ring lines of the twisted-pair telephone line are coupled to tip and ring terminals, respectively, of an HDSL2 port included in the H2TU-R.
It is often the case that the H2TU-R is powered by the twisted-pair telephone line. In one embodiment of a line-powered H2TU-R, the primary winding of the isolation transformer has two halves. One half of the primary winding is coupled to the tip line of the twisted-pair telephone line (via the tip terminal of the HDSL2 port) and the other half of the primary winding is coupled to the ring line of the twisted-pair telephone line (via the ring terminal of the HDSL2 port). The two halves of the primary winding are connected using a capacitor having a relatively high capacitance (for example, on the order of 1.8 microfarads). The power voltage for the H2TU-R is taken across this capacitor.
The H2TU-R typically includes multi-stage protection circuitry intended to protect the signal processing components of the H2TU-R from electrical surges and other conditions that may occur over the twisted-pair telephone line. Typically, such protection circuitry includes a primary protection circuit that includes a pair of gas discharge tubes. One of the gas discharge tubes is coupled between the tip line of the twisted-pair telephone line and chassis ground for the H2TU-R. The other gas discharge tube is coupled between the ring line of the twisted-pair telephone line and chassis ground. The gas discharge tubes typically have a relatively high turn-on voltage (for example, around 1200 volts) and can handle large currents for brief periods of time (for example, from around 5,000 amps to around 10,000 amps for around 1 millisecond). When an electrical surge causes a voltage greater than the turn-on voltage to be established across one of the gas discharge tubes, the gas discharge tube turns on and the current associated with the surge is shunted to chassis ground and away from the other components of the H2TU-R coupled to the twisted-pair telephone line.
The multistage protection circuitry of such an H2TU-R also typically includes a secondary protection circuit. A typical secondary protection circuit includes a pair of fuses or positive thermal coefficient (PTC) thermistors. One fuse or PTC thermistor is in series between the tip line and one half of the primary winding. The other fuse or PTC thermistor is in series between the ring line of the twisted-pair telephone line and the other half of the primary winding. This provides overcurrent protection. The secondary protection circuit also typically includes a pair of transient voltage suppressor devices (for example, a SIDACTOR(R) silicon controlled rectifier (SCR)-type (also referred to as a “thyristor”) transient voltage suppressor device available from Teccor Electronics) to provide overvoltage protection. One transient voltage suppressor is coupled between the tip line of the twisted-pair telephone line and chassis ground. The other transient voltage suppressor is coupled between the ring line of the twisted-pair telephone line and chassis ground. The transient voltage suppressor devices, in one configuration, have turn-on voltages of between approximately 275 volts and approximately 350 volts. The secondary protection circuit is intended to protect the signal processing components of the H2TU-R from electrical surges that are not stopped by the primary protection circuit.
One type of surge that the protection circuitry is intended to protect the signal processing components of the H2TU-R from is an alternating current (AC) power cross surge. An AC power cross surge occurs when one of the lines of the twisted-pair telephone line comes into electrical contact with a 60 hertz (Hz) AC power line. The GR-1089 standard promulgated by Telcordia Labs specifies a first level AC power cross surge test. See, for example, test number 3 from Table 4-7 of the GR-1089 Telcordia standard. In this test, a 60 Hz, 600 volts AC, 1 amp signal is applied across the tip and ring terminals of the HDSL2 port. During each half of each cycle, the capacitor that couples the two halves of the primary winding of the isolation transformer is charged up by current resulting from the incoming surge.
When the voltage across that capacitor exceeds the turn-on voltage for one of the transient voltage suppressors, the transient voltage suppressor turns on and provides a current path to chassis ground. This causes the charged capacitor to discharge through both halves of the primary winding of the transformer and on through the transient voltage suppressor to chassis ground. Due to the relatively low resistance in this current path, a very high current flows through the primary winding of the transformer. This can lead to the induction of a destructive voltage surge on the secondary winding of the transformer, which can potentially damage the signal processing components of the H2TU-R that are coupled to the secondary winding. The high current flowing through the transformer can also cause a gradual heating of the transformer windings, which can cause the transformer to deteriorate and ultimately fail. The process of charging and discharging the capacitor occurs up to about 10 times during each half of each 60 Hz cycle. In other words, during such test, the capacitor is charged and discharged up to about 1200 times during a one second period.